Known hydraulically-actuated fuel injection systems and/or components are shown, for example, in U.S. Pat. No. 5,423,484 issued to Zuo on Jun. 13, 1995 and U.S. Pat. No. 5,492,098 issued to Hafner et al. on Feb. 20, 1996. In these hydraulically-actuated fuel injectors, a spring biased needle check opens to commence fuel injection when pressure is raised by an intensifier piston/plunger assembly to a valve opening pressure. The intensifier piston is acted upon by a relatively high pressure actuation fluid, such as engine lubricating oil, when a solenoid driven actuation fluid control valve opens the injector's high pressure inlet. Injection is ended by deactivating the solenoid to release pressure above the intensifier piston. A return spring biases the intensifier piston back to its retracted position upon the release of pressure above the intensifier piston. This in turn causes a drop in fuel pressure causing the needle check to close under the action of its return spring to end injection.
Engineers have observed that engines using these fuel injectors can sometimes exhibit unsteady behavior when operating at idle conditions. This unsteady behavior often reveals itself as an oscillating rpm at idle conditions, which corresponds to when the fuel injectors are commanded to inject their lowest quantity of fuel. Since the injector's solenoid is energized for such a short amount of time at idle conditions, injection quantities can also vary due to the irregular poppet valve motion. In other words, even reliably consistent short on-times at idle conditions can result in variations between injectors due at least in part to tolerance variations in the components in different injectors. Also, small variations in the commanded on-time can itself cause significant variations in injected fuel quantity at idle conditions.
Rail pressure is preferably reduced at idle in order to reduce excess noise and wasted energy that would result from a higher than needed rail pressure. Also, lower rail pressure results in longer on-times for the same fuel quantity to be injected. Hence, longer on-times at idle will naturally desensitize the system to slight variations in commanded on-times. But rail pressure is generally increased at a rated or cold start condition. The stroke distance of the intensifier piston/plunger assembly at idle is much less than the stroke distance at rated or cold start conditions. Hence, it is desirable to minimize the opposing force on the piston exerted by the piston return spring and lower the rail pressure at idle, yet maximize that force at rated or cold start conditions. At rated or cold start conditions it is desired to reset the piston to its retracted position as soon as possible. Also, under colder conditions more piston return spring force is generally needed because of the increased viscosity of the actuation fluid. At idle conditions, even a relatively weak spring can retract the piston in adequate time for a subsequent injection event.
Selecting a piston return spring that exerts an acceptable force at both idle and a rated or cold start condition is an engineering trade off which results in a less than ideal piston return spring force at either condition. Since unsteady engine performance is very undesirable, especially at idle conditions, there is a motivation to make these hydraulically-actuated fuel injectors less sensitive to fluctuations in rail pressure and/or poppet control valve motion variations.
The present invention is directed to overcoming one or more of the problems as set forth above.